Supports base64-encoded data

Motivation

OpenAPI supports base64-encoded data but to this point OpenAPI Generator
has not (apple/swift-openapi-generator#11).

Modifications

Introduce the `Base64EncodedData` codable type to allow users in the
generator to describe byte types which must be en/de-coded.

Result

Users will be able to describe base64-encoded data as
`OpenAPIRuntime.Base64EncodedData` e.g.
```
public typealias MyData = OpenAPIRuntime.Base64EncodedData
```

Test Plan

Added a round-trip encode/decode test `testEncodingDecodingRoundTrip_base64_success`

Author: rnro

Sources/OpenAPIRuntime/Base/Base64EncodedData.swift

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+public struct Base64EncodedData: Sendable, Codable, Hashable {
+    var data: Foundation.Data // or [UInt8] or ArraySlice<UInt8>
+
+    public init(data: Foundation.Data) {
+        self.data = data
+    }
+
+    public init(from decoder: any Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        let base64EncodedString = try container.decode(String.self)
+        guard let data = Data(base64Encoded: base64EncodedString) else {
+            throw RuntimeError.invalidBase64String(base64EncodedString)
+        }
+        self.init(data: data)
+    }
+
+    public func encode(to encoder: any Encoder) throws {
+        var container = encoder.singleValueContainer()
+        let base64String = data.base64EncodedString()
+        try container.encode(base64String)
+    }
+}

Sources/OpenAPIRuntime/Errors/RuntimeError.swift

@@ -21,6 +21,7 @@ internal enum RuntimeError: Error, CustomStringConvertible, LocalizedError, Pret
     case invalidServerURL(String)
     case invalidExpectedContentType(String)
     case invalidHeaderFieldName(String)
+    case invalidBase64String(String)
 
     // Data conversion
     case failedToDecodeStringConvertibleValue(type: String)
@@ -73,6 +74,8 @@ internal enum RuntimeError: Error, CustomStringConvertible, LocalizedError, Pret
             return "Invalid expected content type: '\(string)'"
         case .invalidHeaderFieldName(let name):
             return "Invalid header field name: '\(name)'"
+        case .invalidBase64String(let string):
+            return "Invalid base64-encoded string: '\(string)'"
         case .failedToDecodeStringConvertibleValue(let string):
             return "Failed to decode a value of type '\(string)'."
         case .unsupportedParameterStyle(name: let name, location: let location, style: let style, explode: let explode):

Tests/OpenAPIRuntimeTests/Base/Test_OpenAPIValue.swift

@@ -12,7 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 import XCTest
-@_spi(Generated) import OpenAPIRuntime
+@_spi(Generated) @testable import OpenAPIRuntime
 
 final class Test_OpenAPIValue: Test_Runtime {
 
@@ -266,4 +266,13 @@ final class Test_OpenAPIValue: Test_Runtime {
         let nestedValue = try XCTUnwrap(nestedDict["nested"] as? Int)
         XCTAssertEqual(nestedValue, 2)
     }
+
+    func testEncodingDecodingRoundTrip_base64_success() throws {
+        print(String(data: testStructData.base64EncodedData(), encoding: .utf8)!.utf8)
+        let encodedData = Base64EncodedData(data: testStructData)
+        XCTAssertEqual(
+            try JSONDecoder().decode(Base64EncodedData.self, from: JSONEncoder().encode(encodedData)),
+            encodedData
+        )
+    }
 }